It is vital to provide a value when assessing
impacts on nature with a view to protecting
them by long-term management . The Darnell system of evaluation is based on the idea
that impacts arising from when humans meet nature can be broken down into twelve
generalizations, or 'principles of environmental impact'. These could be regarded as a
set of working rules which would be the first step in a conservation management. Some
of these generalizations are self- evident, but they are none the less important for that.
They are paraphrased as follows.

A Natural values and the nature of the impact

1 Natural ecosystems are of great value
to society and should not be modified without
good cause.

2 Most human intrusions into natural ecosystems
are likely to be deleterious to that
ecosystem.

3 Ecological impacts always come in groups,
i.e. natural phenomena are interrelated
and impacts are transmitted via species interactions and system connectivity.

B Impact as a function of construction
type and location

4 Each general type of intrusion
results in a characteristic suite of environmental
effects.

5 Each specific type of intrusion induces
a specific set of environmental impacts.

6 There is a regional dimension to all
impacts, i.e. mangroves are not salt marshes.

8 Effects of intrusion are in part predictable,
but in part unpredictable.

9 Different types of intrusion often produce
the same type of response in a system.

10 Severe or prolonged intrusion into
a natural system produces systemic stress, i.e.
total breakdown.

11 Ecosystem responses to severe intrusion
take two forms: a generalized stress
response somewhat unrelated to the nature of the stress agent, and a specific response
which is particular to the nature of the stress agent.

12 Ecosystem responses to stress agents
of various types are not well understood, and
therefore further study is needed if we are to increase our predictive capability and
eventually to minimize the ecological effects of intrusion.

However, such a procedural progression
of considerations still begs a number of
important questions, such as the following. What techniques are available for minimizing
the intrusion/development effects? How far is society prepared to go in ameliorating the
impacts or creating new environments? Is society prepared to forgo immediate
monetary gains for future benefits? What institutional mechanisms, which, of course,
are different in every country and even transcend national boundaries required to achieve
the protection desired? The difficult and vexing questions of establishing markers for the
non-consumptive value of scenic, heritage and historical issues that are still sidestepped.

With respect to the latter aspect, seven
marker categories are presented based on
the different cultural routes taken to evaluate nature. The non-consumptive
benefits of
include scenic, recreational, educational, aesthetic, archaeological, scientific, heritage
and historical benefits that are difficult to define, let alone quantify. These overlap and
intertwine, and consequently a precise definition is not possible or, perhaps, even
desirable.

Non-consumptive values have usually been
considered of secondary importance
compared with the direct consumptive and economic products of environment and the
physical, chemical and biological services that they provide. However, they are
accepted and funded by ordinary people. The range of value categories reflects the fact
that the values are intangible and can be highly personal and subjective. Often values
are placed in society through the media of literature and art, or through scenic or visual-
cultural assessments, and a positive attitude to conservation may be activated through
these doors and windows. Additionally, because there is no way of measuring these
values objectively, it is difficult to compare non-consumptive with consumptive benefits,
or one type of non-consumptive type with another non- consumptive type in the same
kind of environment. The one non- consumptive benefit that can sometimes be
quantified is that of recreation, as the number of people participating, and what they are
prepared to pay for the privilege of that participation in the form of licence fees,
equipment or travel, is a measure of sorts. Thus it is estimated in 1980-1 that 83.2
million Americans spent $14.8 billion on observing and photographing fish and wildlife in
1980-1.